1. Field of the Invention
The present invention is directed to an anesthesia system of the type having a respiratory circuit which conveys respiratory gas to a patient and having a flow meter which measures a flow of fresh respiratory gas, the flow meter requiring periodic calibration.
2. Description of the Prior Art
An anesthesia system has two basic tasks. One is to anesthetize a patient and keep the patient anesthetized. The other is to maintain the patient's respiration during the time the patient is anesthetized. Normally, a respiratory gas containing nitrous oxide (N.sub.2 O), oxygen (O.sub.2) and an anesthetic gas is supplied to the patient. Swedish Published Application 443 722 corresponding to United Kingdom Application 2,226,763 describes one such anesthesia system. This known anesthesia system has a respiratory circuit in which respiratory gas circulates between the patient and a respiratory gas reservoir. An absorber removes carbon dioxide from gas expired by the patient before the gas is returned to the patient in the next breathing cycle. Since the patient always consumes some gas, oxygen and anesthetic gas in particular, fresh respiratory gas is added to the respiratory circuit from a fresh gas source. Surplus gas in the respiratory circuit is removed via a relief valve.
Fresh gas is supplied to the respiratory circuit via rotameters which at a specific pressure supply a specific flow. In this known anesthesia system, oxygen is supplied via one rotameter and nitrous oxide via another rotameter. The gases are then mixed and pass an anesthetic vaporizer before the final mix of respiratory gas is supplied to the respiratory circuit. Since the rotameters and the anesthetic vaporizer operate at relative low pressures and the gas sources contain highly pressurized gas, pressure regulators are arranged between the rotameters and the gas sources in order to limit the pressure.
In certain of the operating modes of an anesthesia system, a specific pressure is desired at the end of expiration, i.e., a PEEP (Positive End Expiratory Pressure). This pressure is selected by the physician and can be varied during the patient's treatment. One problem which could thereby arise is that the flow of fresh respiratory gas selected by the physician may not always be the actual flow of fresh respiratory gas supplied to the respiratory circuit, since the flow from the rotameters depends on the counterpressure in the respiratory circuit. Another problem arising from the use of manual rotameter systems is that the physician sometimes wishes to supply a specific tidal volume to the patient at each inspiration during anesthesia. The anesthesia system is therefore generally controlled so a specific volume of respiratory gas is released from the respiratory gas reservoir and is imposed on the patient during inspiration. This imposed volume of respiratory gas must be compensated for the flow of fresh respiratory gas which is simultaneously supplied to the respiratory circuit. As previously noted, the selected flow of fresh respiratory gas may not be the same as the flow of fresh respiratory gas actually supplied, a circumstance causing problems in the administration of a specific tidal volume to the patient. Even greater problems can arise when the anesthetist wishes to change the flow of fresh respiratory gas to the respiratory circuit. Every time the flow of fresh respiratory gas is changed, control of the tidal volume from the gas reservoir must be changed to ensure that the correct tidal volume continues to be supplied. This is particularly difficult for the physician, since the flow of fresh respiratory gas is calculated in liters/minute and the tidal volume is calculated in liters/breath. Even if the unit "tidal volume" is replaced with the "minute volume" (tidal volume multiplied by the number of breaths/minute) the patient is to inspire, the calculation is not simplified, in principle, for the physician, since breaths are imposed intermittently, and fresh respiratory gas is supplied continuously.
In conventional ventilator/respirator treatment, very accurate regulation and administration of specific tidal volumes to a patient are well-known. For example, the Servo Ventilator 900 C/D from Siemens-Elema AB, Sweden, can be regulated with high accuracy and can supply a virtually exact flow of gas to a patient. In principle, flow regulation is then based on a servo-controlled feedback system in which a flow meter measures flow and a step motor-regulated scissor valve regulates the actual flow. The flow meters which are employed need some form of periodic calibration. For the Servo Ventilator 900 C/D calibration can easily be made. The ventilator only supplies respiratory gas at the specified flow rate during the inspiratory phase and during the expiratory phase the scissor valve completely blocks the flow of gas. The flow meter can therefore be zeroed during the expiratory phase, when no gas flows past the meter. The valve can then supply a correct flow in the next inspiratory phase. In order to further increase the accuracy of the delivered gas flow, the measurement signal from the flow meter is compensated for the current gas mixture before the control signal to the scissor valve is generated. Compensation for the current gas mixture is performed, since the viscosity of the gas mixture affects the mixture's flow.
In the course of the development of a new anesthesia system by Siemens-Elema AB it was decided to utilize the advantages of the Servo Ventilator 900 C/D with regard to its ability to regulate the supply of an almost exact flow. This also conveyed economic benefits in respect to both development costs and production costs. The result was the Servo Anesthesia Circle 985, described in an Operating Manual, AG 0791 0.5, July 1991. In practice, this anesthesia system utilized a slightly modified Servo Ventilator 900 C/D to regulate the supply of fresh respiratory gas to a respiratory circuit. This resulted in an anesthesia system which, in contrast to the known rotameter anesthesia systems, was capable of supplying a well-defined flow of fresh respiratory gas to the respiratory circuit. This anesthesia system was thus able to achieve a number of advantages, since a very efficient ventilator was employed to control the delivery of fresh respiratory gas. For example, the ability to use the anesthesia system for all known anesthesia modes increased. The Servo Anesthesia Circle 985 can, e.g., be used in a completely open anesthesia system, i.e., all the gas expired by the patient is sent to an evacuation unit, and only fresh respiratory gas is delivered to the patient in each inspiration. Since fresh respiratory gas is only supplied during the inspiratory phase, the tidal volume can be easily set and imposed on the patient, with no need for complex calculations by the physician. The anesthesia system can also be used for different kinds of closed and semi-open respiratory circuits.
The use of a known ventilator as the core of the new anesthesia system also imposed certain limitations, inherent in the ventilator used for the anesthesia system, on the new anesthesia system. For example the anesthesia system could only permit the supply of gas during the inspiratory phase, i.e. the flow valve was completely closed during the expiratory phase to zero the flow meter, so that the almost exact flow could be maintained during the inspiratory phase.
Because of the need to calibrate the flow meter, a continuous flow of fresh respiratory gas could not be supplied to the respiratory circuit with this new anesthesia system. The continuous supply of fresh gas generally only occurs in anesthesia systems employing rotameters. It should be noted, that it does not matter to the patient, as regards the induction or safety of anesthesia, whether fresh respiratory gas is supplied continuously or intermittently. Total gas consumption will actually be lower in an intermittent fresh gas delivery system, if a flow of the same magnitude as in continuous delivery is supplied. With continuous supply, however, the respiratory circuit can be flushed more rapidly (in order to empty the respiratory circuit of anesthetic gas when the patient is to be revived or when some other anesthetic is to be administered).
During subsequent development of other ventilator systems, a unique valve system for the Servo Ventilator 300 was designed within Siemens-Elema AB which is described in U.S. Pat. No. 5,265,594. This new valve system has the ability to control extremely accurate flows in a flow range from a few milliliters a minute up to about ten liters a minute. The newly developed valve system is also capable of controlling continuous flows with no loss of its extremely high accuracy. This new valve system would therefore seem well-suited for use in conjunction with the development of anesthesia systems. The newly developed valve system, however, unfortunately requires a specific minimum input pressure for optimum operation. This minimum pressure is higher than the pressure which occurs in the supply of fresh respiratory gas to the respiratory circuit in the known Servo Ventilator 985 anesthesia system. This valve system could therefore not replace the existing valve system for this purpose.
Since intermittent supply and continuous supply of fresh respiratory gas both convey advantages, achieving an anesthesia system giving physicians a choice between these two types of supply, hitherto unavailable in anesthesia systems, would be desirable. A designer of this very special type of anesthesia system therefore face the problem of trying to achieve an anesthesia system offering the physician a choice in the way in which fresh respiratory gas is supplied to the respiratory circuit, i.e. continuously or intermittently. Delivery must also be as accurate as possible, irrespective of the supply option selected by the physician, As the above shows, achieving this has not been possible by combining any of the known anesthesia systems.